Comparative Study of Multi-Physics Generated Small Dipoles in Conducting Media

We investigate dipoles generated by electric current line (TM), electric loop (TE), and mechanically spinning of electret and magnet. While far-zone |HE| of the TE and TM dipole are identical, their near-zone behaviors are drastically different. We present closed form expressions of the Ohmic loss in a spherical lossy shell (SLS) for the first time, which leads to accurate computations and discussions of the article. For electrically small dipole of normalized half dipole-length |ka| ≪1, analytic results show that the radiation efficiency ηr is proportional to |ka| 3 for TM dipole, and |ka| for TE dipole, respectively. Consequently, ηr of TE can be better than TM in two to three orders of magnitude for under seawater communication. The time-domain energy flow velocity (EFV) pattern shows that TM dipole in lossy media is cavity-dominating, while TE/TM in lossless and TE in lossy medium are all radiation-dominating. Numerical results reveal that mechanically spinning dipole is smaller in size and weight but it requires more operation power, compared to its electromagnetic counter-partners. Finally, design, tuning and impedance matching of low-profile TE dipole antenna are outlined.